Model supported rehabilitation and optimization of wastewater treatment plants

Nowadays, the necessity for clean water resources increases due to the progressive increment of population and urbanization, development of industrial production activities and agricultural activities. Therefore, deterioration of water resources and the large amount of polluted water generated in industrialized societies, gives a fundamental importance to wastewater treatment processes in terms of water prevention. National and international regulations enforce the adoption of specific quality indexes for the treated wastewater. Taking into account current environmental problems, it is not unrealistic to believe that this trend will continue. At the same time loads on existing plants are expected to increase due to growth of urban areas. Wastewater generation due to urbanization and industrialization demands more efficient treatment procedures for wastewater which means more effective treatment plants. However, many wastewater treatment plants (WWTPs) are not able to operated properly due to design and operation mistakes. This situation requires rehabilitation and optimization of existing wastewater treatment plants in order to meet the discharge criteria and decrease unaffordable operation costs. Optimizing the operation of existing facilities is becoming more important as effluent criteria become more stringent and available funds for upgrades/expansions become less available. Furthermore, operating costs are now raised as a major concern, especially energy costs. In the scope of the thesis, a modelling study of an existing industrial wastewater treatment plant was carried out and optimization of the plant was aimed by reducing energy consumption. Also, rehabilitation of the plant were considered in order to be able to meet discharge criteria. A textile industry wastewater treatment plant was modelled as a case study which locates in Muratlı Organized Industrial Zone. The facility is one of the largest dyeing plants in Turkey which includes printing operations also. Bleaching and washing activities which are necessary for dyeing of textile products are the main sources of wastewater. Also, domestic usage and water used for cleaning of the facility generate a small part of total wastewater. The wastewater treament plant has 4,500 m3/day treatment capacity and includes biological and chemical treatment units. Average influent flow is about 4,000 m3/day. The plant is designed and operated as an extended aeration activated sludge system for organic material removal and nitrification. Plant design and operation data are considered while modelling study. A commercial wastewater treatment plant simulator that is called GPS-X is used for this case study. GPS-X is a software developed in Canada which provides the platform and input/output capability to utilize activated sludge models and the flexibility to compute numerical solutions. With the use of GPS-X, it is possible to considerably reduce the time required to build, calibrate and simulate treatment processes. Therefore, GPS-X software was used to simulate the plant and system behaviour was evaluated under different operation senarios. By this way, the most applicable operation conditions were determined to increase treatment performance and reduce operating costs. In the scope of the study, metodology of wastewater treatment plant modelling and simulation studies were evaluated in literature and the metodology of the thesis study was determined. First of all, wastewater analysis results were evaluated which were recorded by plant operators. According to effluent wastewater analysis results, WWTP is able to treat organic matter when discharge criteria are considered. However, amonium concentrations in effluent wastewater are above the discharge criteria occasionally. Due to lack of data for modelling of full plant, a monitoring campaign was carried out for 5 days. Characterization analysis of samples were taken from influent, effluent and biological reactor . Total chemical oxygen demand (COD), soluble COD, total kjeldahl nitrogen (TKN), amonium nitrogen (NH4-N), total suspended solids (TSS), volatile suspended solids (VSS) and total phosphorus (TP) concentrations of samples were determined. Plant modelling were done by using Activated Sludge Model no.1 (ASM1) which is a default matematical model in GPS-X software library. ASM1 proposed by the International Water Association (IWA) task group on mathematical modelling for design and operation of biological wastewater treatment. It is one of the most commonly used mathematical description for modelling biological wastewater treatment processes for carbon and nitrogen removal. At the beginning of modelling study definition of preliminary model was done according to monitoring period analysis results. Treatment units were added to model layout according to design dimensions. Wastewater and sludge lines' connections were also defined in preliminary model. Calibration and validation of model were done after definition of preliminary model. Model calibration is defined as the adaptation of a model so that it fitted a certain set of data from the full-scale WWTP. For that reason, it is done by using influent and effluent analysis results in monitoring campaign. Model validation was the next step. The model is considered to be validated when model predictions agree with measured values from an independent dataset within the acceptable tolerance. 10 months average influent and effluent analysis results were used for model validation. After completion of plant modelling, different scenarios were derived in order to reduce energy consumption and improve treatment efficiency of the WWTP with Scenario 1. Within this scope, optimization studies were carried out by considering electricity consumption to decrease operating costs. The current aeration is done by 3 mechanical surface aerators in the plant and 2.0 mg/L dissolved oxygen is supplied to biological reactor. System behaviour and energy consumptions were evaluated under different intermittent aeration implementations with Scenario 1 alternatives. According to Scenario 1 model results, dissolved oxygen concentration supplied with current aeration system in the plant is higher than needed for biological processes. For that reason, capacity of aerators were reduced to supply lower dissolved oxygen concentrations to biological reactor and operated for different time periods than current aeration principle. It is determined that intermittent aeration applications with lower dissolved oxygen concentrations have possitive effect on total nitrogen removal and energy saving. Due to process gain with reduction of TN concentration in effluent wastewater and energy savings, Scenario 1.b is determined as the most applicable alternative for the plant. It is assumed that aerators' capacity is reduced %50 and 1.0 mg/L dissolved oxygen supplied to biological reactor in Scenario 1.b. According to the model results, TN concentrations in effluent wastewater and electricity consumption can be reduced %10 and %28 respectively. It is predicted that yearly operation costs can be reduced about 70,000 TL with the application of Scenario 1.b aeration principle in WWTP. Scenario 1 results show that application of intermittent aeration with controlled lower dissolved oxygen concentrations by considering the increment on influent loads and discharge criteria leads reduction in electricity consumption and affect treatment performans positively. In Scenario 2, rehabilitation of the plant is aimed in order to improve effluent wastewater quality if the WWTP has to meet restricted discharge criteria in future. The current plant's capability was invastigated and applicable process modifications were considered. Within this context, national and international legislation was evaluated and TN discharge criteria of Urban Wastewater Treatment Regulations (UWTR) was choosen for WWTP effluent wastewater quality. The regulations call for advanced treatment methods to eliminate nitrogen and phosphorus in addition to the elimination of organic carbon in sensitive areas. The purpose of the instrument is to protect the environment from the impact of such wastewater discharge. Firstly, population equivalent of WWTP is determined by considering influent wastewater loads. TN discharge criterion is considered as 15 mg/L according to the population equivalent. Due to the existing plant can only perform organic carbon removal and nitrification processes, nitrogen removal is not achieved strictly. It is determined that WWTP's total nitrogen removal performance increases when pre-denitrification activated sludge system is applied. According to Scenario 2 alternatives' model results, the existing plant is not able to achieve TN removal by creating aerobic and anoxic conditions in current biological reactor. For that reason, additional anoxic reactor volumes were added to plant for denitrification process occurrence. According scenario results for different design parameters and reactor volumes, if the biological reactor's volume is increased %33 and 4 internal recycle ratio, TN concentration in effluent wastewater would be below 15 mg/L. In addition to that, nitrification is found the restrictive process in WWTP when influent nitrogen load is higher than average. It is determined that if existing aeration system is operated longer time periods, higher dissolved oxygen concentration can be supplied to biological reactor. Thus, TN concentration fluctuation in effluent can be prevented and nitrogen removal efficiency can be increased. In Scenario 3, WWTP's capability was investigated if influent nitrogen load is increased due to the increment on production capacity of the industrial plant. Applicable process modifications were evaluated in order to improve effluent wastewater quality. In scope of the scenario, effects of increased wastewater flowrate and urea consumption were determined on influent wastewater characterization. Model was run by considering the new wastewater characterization. According to the obtained data relevant with capacity increament from plant operators, TKN concentration of influent is calculated as two times more than current situation. Model results show that COD and NH4-N discharge criteria can be achieved without any process modification in WWTP when influent pollution loads were increased. However, TN concentrations in effluent will be too much and it might be dangerous for receiving water body. In order to achieve nitrogen removal, pre-denitrification activated sludge system was applied to WWTP and effluent wastewater characterization was determined. Model results show that existing biological reactor volume should be increased about %66 and anoxic conditions should be created. Meanwhile, aerobic reactor volume should be same in current system and increased volume will be used for anoxic reactor. Thus, VD/V ratio will be 0.4 in the system. Internal recycle ratio was determined as 4. According to model results, wastewater treatment plant's biological nitrogen removal efficiency can be %59 as maximum.